X-37 can fly in the same orbit as ISS but can they dock?
Yes and no. The current X-37 as flown by the US Air Force is not equipped with any docking ports (PMA, CBM or APAS) as described by @geoffc, nor does it have an attachment for the SSRMS (aka Canadarm) to grab it.
However, Boeing, the manufacturer, have published designs that would permit it to be so equipped at some future time. These plans are described in detail in an article on nasaspaceflight.com. This was part of their COTS proposals. You can get the sense of how this would work from these illustrations:
The ISS has 3 basic docking port types.
- 2 PMA's - used by Shuttle. With addition of LIDS adapter (coming on SpaceX CRS-7 in June 2015) the CST-100 and Dragon Crew will dock to it.
- 2 CBM ports - HTV, Dragon Cargo, Cygnus, and ISS modules berth to these.
- 4 APAS ports - Russian segment has 4 ports. ATV, Soyuz, and Progress use these.
You can see a description of these ports (US Segment on one answer, and Russian segment in the other) in Will the ISS need more docking ports?
Stealing an image from there that sort of shows many examples.
X-37B carries no ports compatible with these systems at this time. No doubt if there was serious demand something could be added. The variety of available ports make that a greater possibility. (The Russians apparently will sell you a Kurs rendezvous system if you want, that is how ATV docks)
But as of now, no.